Amphotericin B lipid complex or amphotericin B multiple-dose administration to rabbits with elevated plasma cholesterol levels: pharmacokinetics in plasma and blood, plasma lipoprotein levels, distribution in tissues, and renal toxicities.

The purpose of the present study was to determine if a relationship exists between the plasma cholesterol concentration, the severity of amphotericin B (AmpB)-induced renal toxicity, and the pharmacokinetics of AmpB in plasma in hypercholesterolemic rabbits administered multiple doses of amphotericin B (AmB) deoxycholate (Doc-AmB) and AmB lipid complex (ABLC). After 7 days of administration of a cholesterol-enriched diet (0.50% [wt/vol]) or a regular rabbit diet, each rabbit was administered a single intravenous bolus of Doc-AmB (n = 8) or ABLC (n = 10) (1.0 mg/kg of body weight) daily for 7 consecutive days (a total of eight doses). Blood samples were obtained daily before and 24 h after the administration of each dose and serially thereafter following the administration of the last dose for the assessment of pharmacokinetics in plasma, kidney toxicity, plasma lipoprotein levels, and drug distribution in tissue. The pharmacokinetics of AmB in blood following the administration of ABLC were also determined in rabbits fed cholesterol-enriched and regular diets (n = 3 each group). Before drug treatment, cholesterol-fed rabbits demonstrated marked increases in total, low-density lipoprotein (LDL), and triglyceride-rich lipoprotein (TRL) cholesterol levels in plasma compared with the levels in rabbits on a regular diet. No significant differences in total plasma triglyceride levels were observed. Significant increases in plasma creatinine levels were observed in rabbits fed a cholesterol-enriched diet (P < 0.05) and rabbits fed a regular diet (P < 0.05) when administered AmB. However, the magnitude of this increase was twofold greater in rabbits fed a regular diet than in rabbits fed a cholesterol-enriched diet. An increase in plasma creatinine levels was observed only in rabbits on a cholesterol-enriched diet administered ABLC. The pharmacokinetics of AmB were significantly altered in rabbits on a cholesterol-enriched diet administered Doc-AmB or ABLC compared to those in rabbits on a regular diet administered each of these compounds. The pharmacokinetics of AmB in blood were significantly different following ABLC administration but not following Doc-AmB administration in both rabbits fed cholesterol-enriched diets and rabbits fed regular diets compared to their corresponding pharmacokinetics in plasma. An increased percentage of AmB was recovered in the TRL fraction when Doc-AmB was administered to rabbits fed a cholesterol-enriched diet than when it was administered to rabbits fed a regular diet. Furthermore, an increased percentage of AmB was recovered in the LDL and TRL fractions when ABLC was administered to rabbits fed a cholesterol-enriched diet rabbits fed a regular diet. These findings suggest that an increase in plasma cholesterol levels modifies the pharmacokinetics of AmB and renal toxicity following the administration of multiple intravenous doses of Doc-AmB and ABLC.

Lipoprotein isolation and analysis from serum by preparative ultracentrifugation.

Plasma lipoproteins are a heterogeneous population of soluble, macromolecular aggregates of lipids and proteins. They are responsible for the transport of waterinsoluble nutrients through the vascular and extravascular fluids from their site of synthesis or absorption to peripheral tissues (1,2). These hydrophobic nutrients (triacylglycerols [TGs] and cholesteryl esters [CEs]) are delivered from the liver and intestine to other tissues in the body for storage or catabolism in the production of energy. Lipoproteins are also known to be involved in other biological processes, including coagulation and tissue repair as well as immune reactions (3,4).

Direct neuronal interactions between the duodenum and the sphincter of Oddi.

The sphincter of Oddi (SO) is a complex structure that must function in coordination with the motor activities of the gallbladder and the duodenum. It is now clear that a neural circuit exists between the duodenum and the SO, and it is likely that this network is largely responsible for the regulation of SO motility. Recent studies have demonstrated that this circuit provides excitatory cholinergic input to SO ganglia that can be activated by electrical stimulation of the duodenal mucosa, distention of the duodenum, and increased motor activity of the duodenum.

Supervision for Practicing Genetic Counselors: An Overview of Models.

Supervision has traditionally been a concept that refers primarilyto a means by which to train genetic counseling students. However, supervision-redefined-can afford anextraordinary opportunity for practicing genetic counselors to regularly update and enhancetheir counseling skills. This paper provides a new definition of supervision for experiencedgenetic counselors and discusses the process and advantages of different types ofsupervision. The leader-led peer supervision group will be highlighted as especiallyamenable to the needs of genetic counselors for a collegial forum in which to discuss thepsychosocial components of their work. Immediately following, in a separate paper, will bea description of a currently ongoing supervision group for practicing genetic counselors.

A Leader-Led Supervision Group as a Model for Practicing Genetic Counselors.

With the stage set by the overview of supervision models in the previous paper, thispaper now presents the development and evolution of a currently ongoing leader-ledsupervision group for experienced genetic counselors. I discuss the procedures forgetting started; the creation and maintenance of the contract; typical issues and themesconsidered; the format for case presentation; and the overall growth of the group and itsmembers.

Preferential distribution of amphotericin B lipid complex into human HDL3 is a consequence of high density lipoprotein coat lipid content.

The purpose of this study was to determine the plasma lipoprotein (LP) distribution of amphotericin B (AmpB) and amphotericin B lipid complex [ABLC; Abelcet composed of dimyristoyl phosphatidylcholine (DMPC) and dimyristoyl phosphatidylglycerol (DMPG)] and define the relationship between LP lipid concentration and composition and the distribution of AmpB and ABLC in human plasma with varying total and lipoprotein cholesterol and triglycerides. AmpB and ABLC at a concentration of 20 microg amphotericin B/mL were incubated in plasma obtained from different human subjects (n = 7) for 60 min at 37 degrees C. Following these incubations plasma samples were separated into their high-density lipoprotein (HDL), triglyceride-rich lipoprotein (TRL; which contains very low-density lipoproteins and chylomicrons), low-density lipoprotein (LDL), and lipoprotein-deficient (LPDP) fractions by density-gradient ultracentrifugation (UC) and each fraction was assayed for AmpB using high-pressure liquid chromatography (HPLC). The HDL fraction was further separated into its HDL3 and HDL2 subclasses by UC and assayed for AmpB using HPLC. Separation of HDL into its subclasses was confirmed by gel electrophoresis. To assess the influence of modified lipoprotein concentrations and lipid composition on the plasma distribution of AmpB and ABLC, these compounds were incubated in plasmas from human subjects with varying total and lipoprotein lipid concentrations. In addition, to demonstrate that alterations in HDL lipid composition influence the plasma distribution of ABLC, ABLC (20 microg amphotericin B/mL) was incubated in plasma pretreated with dithionitrobenzoate (DTNB, a compound which inhibits lecithin:cholesterol acyltransferase conversion of HDL3 free cholesterol to esterified cholesterol) 18 h prior to the experiment or in untreated plasma for 60 min at 37 degrees C. Total plasma and lipoprotein cholesterol (TC), free cholesterol (fC), esterified cholesterol (CE), triglyceride (TG), phospholipid (PL), and protein (TP) concentrations in each human sample were determined by enzymatic assays. When AmpB was incubated in human plasmas of varying lipid concentrations, the majority of the drug was recovered in the LPDP fraction. However, the majority of AmpB was recovered in the HDL3 fraction following the incubation of ABLC. Differences in lipid coat content (fC and PL) carried by HDL influenced the distribution of ABLC within plasma of different human subjects. These findings were confirmed by the DTNB treatment experiments. These findings suggest that the association of AmpB with DMPC and DMPG to form drug-lipid complexes modifies the plasma distribution of the AmpB. In addition, the distribution of ABLC among plasma lipoproteins of different human subjects is defined by the HDL lipid coat content and is possibly an important consideration when evaluating the pharmacokinetics, toxicity, and activity of these compounds following administration to humans with differing plasma lipid concentrations.

Duodenal neurons provide nicotinic fast synaptic input to sphincter of Oddi neurons in guinea pig.

We have investigated the existence of neural connections between the duodenum and the sphincter of Oddi (SO). Stimulation of duodenal myenteric fiber bundles elicited synaptic responses in SO neurons, which included nicotinic fast excitatory postsynaptic potentials (EPSPs), slow EPSPs, and alpha(2)-adrenoreceptor-mediated inhibitory postsynaptic potentials. After 48 h in organ culture, when extrinsic fibers had diminished, only the fast EPSPs persisted. Duodenal mucosal stimulation also elicited nicotinic fast EPSPs in SO neurons. There was no association between the SO neurons that received duodenal input and their chemical coding. A reciprocal projection also exists from the SO to the duodenum. In acute and cultured preparations, duodenal myenteric stimulation caused antidromic responses in 20% of SO neurons. Furthermore, 45.6 +/- 10.5 neurons in SO ganglia were retrogradely labeled from dye application sites in the duodenum. It is proposed that bidirectional neural communication occurs between the duodenum and the SO and that duodenal neurons provide excitatory fast synaptic input to SO neurons through a reflex that can be activated at the duodenal mucosa.

Neuropeptide Y (NPY) expression is increased in explanted guinea pig parasympathetic cardiac ganglia neurons.

While expression of neuropeptides by sympathetic neurons is altered by decentralization and axotomy, it is not known whether similar experimental paradigms also modulate the chemical phenotype of parasympathetic cardiac ganglia neurons. The present study tested whether guinea pig parasympathetic neuron neuropeptide Y (NPY) expression was altered when cardiac ganglia preparations were maintained as organ explants in the presence or absence of colchicine. Two experimental approaches were used to examine NPY expression. First, immunocytochemical techniques were used to quantitate numbers of neurons within the cardiac ganglia exhibiting NPY-immunoreactivity; second, reverse transcription PCR was used to examine proNPY mRNA expression. In control cardiac ganglia preparations, approximately 4% of ganglia neurons exhibited NPY-immunoreactivity. The percentage of NPY-immunopositive neurons in 30- and 72-h explanted cardiac ganglia preparations, maintained in the absence of colchicine, increased to 11 and 16%, respectively. Colchicine treatment of explanted preparations further increased the percentage of NPY-positive ganglia cells 24% (30 h) and 32% (72 h). All NPY-immunoreactive neurons from control ganglia and explanted ganglia were choline acetyltransferase(ChAT)-immunoreactive, indicating retention of the cholinergic phenotype. ProNPY mRNA also was increased following ganglia explantation, consistent with the increase in the numbers of NPY-immunoreactive neurons. NPY transcripts were further increased after 30 h, but not after 72 h in colchicine-treated, explanted cardiac ganglia preparations. These results demonstrate that NPY expression is altered in explanted cardiac ganglia preparations, providing evidence that the chemical phenotype of parasympathetic cardiac neurons can be modulated.

Specific IgG response to monomeric and polymeric diphenylmethane diisocyanate conjugates in subjects with respiratory reactions to isocyanates.

BACKGROUND: Isocyanates are a frequent cause of occupational asthma and can also induce hypersensitivity pneumonitis. OBJECTIVES: It is still unclear whether antibodies to diphenylmethane diisocyanate (MDI), which are elicited in some subjects with these conditions, are specific for this type of isocyanate. Moreover, preparation of conjugates to human serum albumin (HSA) with the polymeric formulation rather than monomeric MDI might result in improved detection of antibodies. METHODS: We addressed these issues by testing the sera of 13 subjects with asthma (n = 12) and hypersensitivity pneumonitis (n = 1) induced by MDI (n = 4 or 5, see below) by comparing them with sera obtained from subjects with occupational asthma caused by toluene diisocyanate (TDI; n = 5) and hexamethylene diisocyanate (HDI; n = 2). Conjugate preparations were compared by using SDS-PAGE, absorbance spectral analysis, and isolectric focusing. Immunologic screening was done by ELISA. RESULTS: Specific IgG antibodies that recognize MDI-HSA conjugates were detected in all but 1 of the MDI-exposed workers and could not be found in TDI-exposed and HDI-exposed workers. The levels of specific IgG antibodies were more elevated when tested against the HSA conjugates formed with polymeric MDI compared with the HSA conjugates formed with monomeric MDI. CONCLUSION: This study shows that specific IgG antibodies to MDI appear to be specific for MDI without cross-reactivity with TDI and HDI and higher by use of polymeric rather than monomeric MDI-HSA test antigens.

Pharmacokinetics, distribution in serum lipoproteins and tissues, and renal toxicities of amphotericin B and amphotericin B lipid complex in a hypercholesterolemic rabbit model: single-dose studies.

The purpose of this study was to determine if a relationship exists among total serum and lipoprotein cholesterol concentration, the severity of amphotericin B (AmpB)-induced renal toxicity, and the serum pharmacokinetics of AmpB in hypercholesterolemic rabbits administered AmpB and AmpB lipid complex (ABLC). After 10 days of cholesterol-enriched diet (0.50% [wt/vol]) or regular rabbit diet (control), each rabbit was administered a single intravenous bolus of AmpB or ABLC (1.0 mg/kg of body weight). Blood samples were obtained before administration and serially thereafter for the assessment of serum pharmacokinetics, kidney toxicity, and serum lipoprotein distribution. Rabbits were humanely sacrificed after all blood samples were obtained, and tissues were harvested for drug analysis. Before drug treatment, cholesterol-fed rabbits demonstrated marked increases in total serum cholesterol and low-density lipoprotein (LDL) cholesterol levels compared with levels in rabbits on a regular diet. No significant differences in triglyceride levels were observed. A significant increase in serum creatinine levels was observed in cholesterol-fed and regular diet-fed rabbits administered AmpB. However, the magnitude of this increase was 2.5-fold greater in cholesterol-fed rabbits than in regular diet-fed rabbits. No significant differences in triglyceride levels were observed. A significant increase in serum creatinine levels was observed in cholesterol-fed and regular diet-fed rabbits administered ABLC. Whereas AmpB pharmacokinetics were significantly altered in cholesterol-fed rabbits administered free AmpB, similar AmpB pharmacokinetics were observed in both rabbit groups administered ABLC. Renal AmpB levels were significantly increased in cholesterol-fed rabbits administered AmpB compared with those in all other groups. Hepatic and lung AmpB levels were elevated in cholesterol-fed rabbits administered free AmpB compared to controls. In addition, hepatic, lung, and spleen AmpB levels were significantly decreased in cholesterol-fed rabbits administered ABLC compared to controls. An increased percentage of AmpB was recovered in LDL-very-low-density lipoprotein fraction when free AmpB was administered to cholesterol-fed rabbits compared with those in all other groups. These findings suggest that increases in cholesterol, specifically, LDL cholesterol levels, modify the disposition and renal toxicity of free AmpB. However, the pharmacokinetics and renal toxicity of ABLC were independent of elevations in total and LDL cholesterol levels.

Glycosylated haemoglobin and hypertension arising in pregnancy.

We analysed a database of glycosylated haemoglobin (HbA1) in nondiabetic pregnant women to investigate the relation between glucose metabolism in the first and second trimesters and hypertensive complications of pregnancy. From a total of 1334 women, 13 had pre-existing hypertension, 225 developed gestational hypertension and 51 developed pre-eclampsia. At 28 weeks of gestation, the women who subsequently developed gestational hypertension had a significantly higher mean HbA1 than those who remained normotensive (6.33 vs 6.17%, P < 0.02). This difference remained significant after correcting for the effects of age and body mass index (regression coefficient 0.11, SE 0.06, P = 0.05). In contrast, there were no significant differences in HbA1 between the women with pre-eclampsia and their normotensive counterparts. This provides indirect evidence to support our hypothesis that gestational hypertension is associated with insulin resistance but pre-eclampsia is not.

Expression and physiological actions of neuropeptide Y in guinea pig parasympathetic cardiac ganglia.

Guinea pig atrial whole mount preparations containing the parasympathetic cardiac ganglia were used to establish the expression, distribution and actions of neuropeptide Y (NPY) in atrial tissues. NPY-immunoreactive fibers densely innervated the atrial myocardium and blood vessels. Fibers containing NPY also innervated intrinsic parasympathetic cardiac neurons. Four percent of the cardiac neurons, identified using microtubule associated protein-2 antiserum, were NPY-positive. An endogenous source of NPY was confirmed with reverse transcription PCR which demonstrated the presence of proNPY mRNA. Sixty percent of the parasympathetic cardiac neurons were hyperpolarized by local application of NPY. NPY also decreased the amplitude and duration of the action potential after hyperpolarization in 60% of the neurons and decreased the fast excitatory postsynaptic potential in about 50% of the cells. These observations indicate that NPY is anatomically positioned to directly alter the output of the parasympathetic cardiac ganglia either by hyperpolarizing the cardiac neurons or by decreasing the fast synaptic input which drives individual neurons.

Duodenal sensory neurons project to sphincter of Oddi ganglia in guinea pig.

Retrograde labeling of duodenum-sphincter of Oddi (SO) preparations in vitro with the carbocyanine dye DiI revealed that duodenal neurons project to the SO. The duodenum-SO-projecting neurons were immunoreactive (IR) for choline acetyltransferase but not nitric oxide synthase or calretinin, indicating that this is a cholinergic projection and that this pathway is distinct from the circuitry involved in the ascending limb of the peristaltic reflex. Approximately 20% of the duodenum-SO projection neurons were IR for calbindin. Calbindin-IR nerves within SO ganglia degenerated when the SO was maintained in organ culture alone, but persisted when the SO was cultured with the duodenum intact. Therefore, SO ganglia are a target of the calbindin-positive duodenum-SO projection. Because calbindin is a marker of intrinsic sensory neurons that have processes that pass to the mucosa, these neurons are in position to detect the release of a compound from the mucosa and signal its release to SO ganglia. When applied to retrogradely labeled neurons, cholecystokinin (CCK) elicited a prolonged depolarization, indicating that duodenum-SO-projecting neurons could be capable of detecting CCK released from the mucosa. It is proposed that the role of the intrinsic sensory neurons that project to the SO may be to signal the postprandial release of CCK, thus providing an instruction to decrease SO resistance and facilitate the flow of bile into the duodenum.

Identification of the cholinergic neurons in guinea-pig sphincter of Oddi ganglia.

The muscular tone of the sphincter of Oddi (SO) can be up- or down-regulated by neurons that lie within ganglia in the wall of the tissue. Previous studies have demonstrated that neurons in the ganglia of the guinea-pig SO can be classified into two major populations, one of which expresses tachykinins and enkephalin and another which expresses nitric oxide synthase. Although results of previous pharmacological studies indicate that acetylcholine is released in the SO, the neurons that express this neurotransmitter have not previously been identified. This study was conducted to establish which neurons in the ganglia of the guinea-pig SO are cholinergic by examining the distribution of choline acetyltransferase (ChAT) immunoreactivity, since the enzyme, ChAT is necessary for acetylcholine synthesis. Choline acetyltransferase immunoreactivity was intense and widespread in the ganglionated plexus of the SO. ChAT-immunoreactive nerve fibers were present in ganglia, interganglionic fiber bundles and in the circular muscle layer. Neurons that were immunoreactive for ChAT comprised about 69% of the population and most of these neurons were also tachykinin-immunoreactive. Co-expression of ChAT and nitric oxide synthase was not observed in nerve cell bodies or nerve fibers. Data from this study support the concept that SO ganglia are largely made up of two populations of neurons, one excitatory and the other inhibitory, on the basis of their chemical coding. The excitatory neurons are cholinergic and co-express tachykinin and opiate peptides and the inhibitory neurons are ChAT-negative and express nitric oxide synthase.

Analysis of the reactivity of [14C]toluene diisocyanate (TDI) in an isolated, perfused lung model.

An isolated, perfused, guinea pig lung model was used to investigate the molecular events which occur when a 14C-labeled TDI vapor reaches the airways. Exposure concentrations of 0.2 and 0.7 ppm were tested. Perfusate composition included: Krebs Ringer buffer only, as well as buffer containing either guinea pig serum albumin, human serum albumin, or diluted guinea pig plasma. Radioactivity was detected in the perfusate within minutes of exposure, and following a delay, increased linearly. The rate of uptake was dependent on TDI concentration and the composition of the perfusate. Biochemical characterization of the state of the 14C-labeled material in the perfusate was performed. The distribution between low and high molecular weight reaction products was determined by molecular sieve fractionation and varied as a function of perfusate composition but no variability was observed as a function of time during the 45 min of exposure. An increase in nucleophile concentration in the perfusate was associated with both a higher percentage of conjugated products (from 15% with buffer only to 45% with diluted guinea pig plasma) and an increase in the rate of TDX uptake (from 0.5 microns Eq/min with buffer alone to 0.1 micrograms Eq/min with diluted GPSA as perfusate at 0.7 ppm). GC-MS analysis of the samples for free TDA, before and after acid hydrolysis, showed that the low molecular weight product(s), which represented from 55-85% of the circulating radioactivity, was composed of hydrolyzable and non-hydrolyzable conjugates and metabolites with approximately 4% of the label associated with free TDA. Although the distribution between high and low molecular weight species varies, this result is analogous to the findings from in vivo studies and suggests that the isolated, perfused lung (IVPL) system may be a useful tool in investigating the molecular mechanisms of isocyanate-induced disease and metabolic activity of the lung.

Distribution and reactivity of inhaled 14C-labeled toluene diisocyanate (TDI) in rats.

Inhalation exposure to toluene diisocyanate (TDI) can result in a variety of airway diseases. Concern has been expressed that a putative carcinogenic potential of TDI exists as a result of the formation of toluenediamine (TDA) by hydrolysis of the isocyanate in the body. Results from long-term bioassays (TDI inhalation versus gavage in rats and mice) are contradictory and discrepancies do exist concerning the interpretation of adverse effects. This study was performed to analyze the distribution and reactivity of radioactively-labeled TDI using vapor exposure in a rat model system. Rats were exposed to 14C-TDI vapors at concentrations ranging from 0.026 to 0.821 ppm for 4 h. All tissues examined showed detectable quantities of radioactivity, with the airways, gastrointestinal system and blood having the highest levels which increased with exposure concentration. The concentration of radioactivity in the bloodstream after exposure was linear with respect to dose. The majority (74-87%) of the label associated with the blood was recovered in the plasma, and of this, 97-100% of the 14C existed in the form of biomolecular conjugates. Analysis of stomach contents shows that the majority of the label is also associated with high (> 10 kDa) molecular weight species. While a larger percentage (28%) of the label is found in the low molecular weight fraction relative to blood, this low molecular weight labeled material represents at least eight different components. Thus, over the vapor exposure concentrations and time tested, it appears that conjugation is the predominant reaction and that free TDA is not a primary in vivo reaction product under the conditions tested.

Mouse Clara cell 10-kDa (CC10) protein: cDNA nucleotide sequence and molecular basis for the variation in progesterone binding of CC10 from different species.

A protein similar to the rat Clara cell 10-kDa protein (CC10) was isolated from mouse lung homogenate by conventional chromatography. cDNA for the mouse CC10 protein was identified in the mouse lung cDNA library by using radiolabeled rat CC10 cDNA as the probe. The isolated cDNA was sequenced and the deduced primary amino acid sequence was compared to the known sequences of rabbit and hare uteroglobins and human and rat CC10 proteins. The cDNA sequence was confirmed by N-terminal amino acid sequencing of the purified protein. The purified mouse CC10 was tested for its ability to bind progesterone, and the binding was found to be 27% lower than rat CC10 and 48% lower than rabbit uteroglobin. The relative binding of mouse, rat, and human CC10 may reflect subtle structural perturbations. The only notable difference between mouse and rat CC10 is in the beta bend between helices 1 and 2, at residue 16. This difference also exists between rat and human CC10. The mouse CC10 sequence compares favorably with human CC10, which does not bind progesterone; however, the mouse CC10 does not contain M60, which has been proposed to block the binding of progesterone with human CC10. The wide variation in progesterone binding among this family of proteins casts doubt on the importance of such binding as a physiologic function.

Autoradiographic analyses of guinea pig airway tissues following inhalation exposure to 14C-labeled methyl isocyanate.

Through the use of radioactively labeled methyl isocyanate (MIC), the deposition, penetration, and clearance of this highly reactive compound in the airway at the tissue and cellular levels have been directly examined. Guinea pigs were exposed to 14C-MIC vapors at concentrations ranging from 0.38 to 15.2 ppm for periods of 1-6 hr. Solubilization of tissues from these animals showed the airway tissues to have the highest level of radioactivity. In the nasal region, 14C deposition, as monitored by histoautoradiography, was limited to the epithelial layer, was related to dose, and was dependent on the specific epithelial cell type. The squamous epithelium was minimally labeled on the surface and the label did not penetrate the cell layer. However, radioactivity was detected throughout the entire nasal respiratory epithelial layer. The lack of nasal deposition in tracheotomized animals demonstrated that the 14C accumulation at this site was due to the scrubbing action of the nasal region with no contribution from blood recirculation. Cellular localization in the tracheobronchial region showed epithelial and subepithelial deposition in a dose-dependent manner with accumulation of the label at the subepithelial region. Radioactivity penetrated to the level of the terminal bronchiole but was not detected in the alveolar region. The persistence of airway radioactivity over the 48-hr postexposure period monitored suggests the covalent modification of airway macromolecules. Despite its broad specificity and high reactivity, MIC undergoes selective reactions in the airways which are dependent on respiratory region and cell type.